www.nature.com/scientificreports
OPEN
Received: 10 March 2017 Accepted: 10 July 2017 Published: xx xx xxxx
Repetitive motor cortex stimulation reinforces the pain modulation circuits of peripheral neuropathic pain Myeounghoon Cha1, Sun Woo Um1,2, Minjee Kwon1,2, Taick Sang Nam1 & Bae Hwan Lee 1,2 Recent evidence indicates that motor cortex stimulation (MCS) is a potentially effective treatment for chronic neuropathic pain. However, the neural mechanisms underlying the attenuated hyperalgesia after MCS are not completely understood. In this study, we investigated the neural mechanism of the effects of MCS using an animal model of neuropathic pain. After 10 daily sessions of MCS, repetitive MCS reduced mechanical allodynia and contributed to neuronal changes in the anterior cingulate cortex (ACC). Interestingly, inhibition of protein kinase M zeta (PKMζ), a regulator of synaptic plasticity, in the ACC blocked the effects of repetitive MCS. Histological and molecular studies showed a significantly increased level of glial fibrillary acidic protein (GFAP) expression in the ACC after peripheral neuropathy, and neither MCS treatment nor ZIP administration affected this increase. These results suggest that repetitive MCS can attenuate the mechanical allodynia in neuropathic pain, and that the activation of PKMζ in the ACC may play a role in the modulation of neuropathic pain via MCS. When the sensory nervous system is affected by injury or disease, it often leads to a sense of numbness or lack of sensation. Neuropathic pain is thought to be associated with these types of abnormal peripheral and central nerve problems, which can lead to the development of a chronic neuropathic pain state. Recent studies have suggested that the development of neuropathic pain involves not only neurons but also glial cells, including astrocytes and microglia, which interact with neurons and thereby modulate pain transmission under pathophysiological conditions1–3. Increased peripheral sensory nerve activity induces multiple trans-synaptic modifications that extend to the central nervous system (CNS). Furthermore, persistent chronic pain induces significant functional and structural changes in the nervous system4. These new synaptic formations in the CNS underlie the plasticity of neurons. Neuropathic pain-induced synaptic plasticity has been documented in many cortical regions associated with pain perception5, 6. A recent report demonstrated that an elevation in astrocytic activity initiates increased synaptic remodeling in the brain7. The strengthening of the synaptic interactions between specific cells in the CNS affected the formation of a new memory. When LTP is generated, the number of surrounding astrocytes increases to supply sufficient energy for the newly generated synapses6. Therefore, the degree of astrocytic hypertrophy, which of can be determined by measuring the number of and assessing the shape of astrocytes, can be used as direct or indirect evidence of activated synaptic plasticity2, 8. Since its initial publication in the early 1990s, epidural motor cortex stimulation (MCS) using surgically implanted electrodes has been shown to be capable of producing long-term analgesia in approximately half of the patients with chronic neuropathic pain resistant to medication9. MCS is easier to implement for pain modulation than other surgical methods, such as direct nerve stimulation and neurectomy, and it can be considered an alternative treatment for pain control10. Recent studies have reported that pain relief occurs progressively after the onset of MCS and persists after the stimulation has stopped11–13. This effect of MCS can last from minutes to days in some patients and suggests that MCS could potentially serve as a therapy for the treatment of resistant neuropathic pain14, 15. Furthermore, repetitive stimulation of the motor cortex induces homeostatic plasticity as a means of stabilizing the properties of neuronal circuits in the brain16, 17. However, the underlying mechanism of MCS in pain modulation is poorly understood. 1 Department of Physiology, Yonsei University College of Medicine, Seoul, 03722, Republic of Korea. 2Brain Korea 21 PLUS Project for Medical Science, Yonsei University College of Medicine, Seoul, 03722, Republic of Korea. Correspondence and requests for materials should be addressed to B.H.L. (email: [email protected])
Scientific ReporTS | 7: 7986 | DOI:10.1038/s41598-017-08208-2
1
www.nature.com/scientificreports/ Recent studies have described the anterior cingulate cortex (ACC) as a cortical area in the brain involved with pain, possibly including both perception and modulation via neural plasticity18, 19. However, despite studies demonstrating that ACC projection is deeply related to the motor cortex, the underlying mechanism of MCS in pain modulation is poorly understood18. The activation of astrocytes and/or astrogliosis is one of the changes that has been observed in the ACC during chronic pain induced by nerve injury20, 21. In addition, nerve injury manifests as an increased expression of astrocytic markers, such as glial fibrillary acidic protein (GFAP), in the ACC8. Evidence from previous reports shows that astrocytes perform various functions, including the biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to nervous tissue, maintenance of extracellular ion balance, and a role in the repair and scarring process of the brain and spinal cord following traumatic injuries22–24. Astrocytes are the most numerous non-neuronal cells in the brain involved in the modulation of neuronal activities, such as extracellular and synaptic cleft neurotransmitter level regulation and the release of neuroactive molecules18. Moreover, recent studies have suggested that astrocytes play an important role in the synaptic cleft and in the interactions between pain-transmitting neurons and other neurons5, 7. This is because astrocytes can detect neuronal activity and release chemical transmitters, which in turn control synaptic activity19, 22. Thus, we hypothesized that repetitive MCS may induce analgesic effects on chronic neuropathic pain by inducing a modification in synaptic connections, resulting in the attentuation of mechanical hypersensitivity in neuropathic pain. The aim of this study was to examine the altered synaptic connection of the ACC in a rat model of neuropathic pain and MCS-induced behavioral modifications. Our results implicate the potential role of cortical astrocytes and ACC structural synaptic plasticity in mechanical hyperalgesia and contribute to the understanding of the mechanism of MCS-induced analgesic effects in an animal model of neuropathic pain.
Results
Repetitive MCS attenuated pain behavior associated with neuropathic pain. We have previously shown that tibial and sural nerve ligation in rats produced mechanical and thermal hypersensitivity25. The withdrawal threshold and latency induced by electrical von Frey stimulation were measured in peripheral nerve-injured rats. As shown in Fig. 1A, the withdrawal thresholds in neuropathic pain (NP) rats significantly decreased after nerve injury. The measured values of the withdrawal response in NP rats were significantly reduced from the day following nerve injury (P
OPEN
Received: 10 March 2017 Accepted: 10 July 2017 Published: xx xx xxxx
Repetitive motor cortex stimulation reinforces the pain modulation circuits of peripheral neuropathic pain Myeounghoon Cha1, Sun Woo Um1,2, Minjee Kwon1,2, Taick Sang Nam1 & Bae Hwan Lee 1,2 Recent evidence indicates that motor cortex stimulation (MCS) is a potentially effective treatment for chronic neuropathic pain. However, the neural mechanisms underlying the attenuated hyperalgesia after MCS are not completely understood. In this study, we investigated the neural mechanism of the effects of MCS using an animal model of neuropathic pain. After 10 daily sessions of MCS, repetitive MCS reduced mechanical allodynia and contributed to neuronal changes in the anterior cingulate cortex (ACC). Interestingly, inhibition of protein kinase M zeta (PKMζ), a regulator of synaptic plasticity, in the ACC blocked the effects of repetitive MCS. Histological and molecular studies showed a significantly increased level of glial fibrillary acidic protein (GFAP) expression in the ACC after peripheral neuropathy, and neither MCS treatment nor ZIP administration affected this increase. These results suggest that repetitive MCS can attenuate the mechanical allodynia in neuropathic pain, and that the activation of PKMζ in the ACC may play a role in the modulation of neuropathic pain via MCS. When the sensory nervous system is affected by injury or disease, it often leads to a sense of numbness or lack of sensation. Neuropathic pain is thought to be associated with these types of abnormal peripheral and central nerve problems, which can lead to the development of a chronic neuropathic pain state. Recent studies have suggested that the development of neuropathic pain involves not only neurons but also glial cells, including astrocytes and microglia, which interact with neurons and thereby modulate pain transmission under pathophysiological conditions1–3. Increased peripheral sensory nerve activity induces multiple trans-synaptic modifications that extend to the central nervous system (CNS). Furthermore, persistent chronic pain induces significant functional and structural changes in the nervous system4. These new synaptic formations in the CNS underlie the plasticity of neurons. Neuropathic pain-induced synaptic plasticity has been documented in many cortical regions associated with pain perception5, 6. A recent report demonstrated that an elevation in astrocytic activity initiates increased synaptic remodeling in the brain7. The strengthening of the synaptic interactions between specific cells in the CNS affected the formation of a new memory. When LTP is generated, the number of surrounding astrocytes increases to supply sufficient energy for the newly generated synapses6. Therefore, the degree of astrocytic hypertrophy, which of can be determined by measuring the number of and assessing the shape of astrocytes, can be used as direct or indirect evidence of activated synaptic plasticity2, 8. Since its initial publication in the early 1990s, epidural motor cortex stimulation (MCS) using surgically implanted electrodes has been shown to be capable of producing long-term analgesia in approximately half of the patients with chronic neuropathic pain resistant to medication9. MCS is easier to implement for pain modulation than other surgical methods, such as direct nerve stimulation and neurectomy, and it can be considered an alternative treatment for pain control10. Recent studies have reported that pain relief occurs progressively after the onset of MCS and persists after the stimulation has stopped11–13. This effect of MCS can last from minutes to days in some patients and suggests that MCS could potentially serve as a therapy for the treatment of resistant neuropathic pain14, 15. Furthermore, repetitive stimulation of the motor cortex induces homeostatic plasticity as a means of stabilizing the properties of neuronal circuits in the brain16, 17. However, the underlying mechanism of MCS in pain modulation is poorly understood. 1 Department of Physiology, Yonsei University College of Medicine, Seoul, 03722, Republic of Korea. 2Brain Korea 21 PLUS Project for Medical Science, Yonsei University College of Medicine, Seoul, 03722, Republic of Korea. Correspondence and requests for materials should be addressed to B.H.L. (email: [email protected])
Scientific ReporTS | 7: 7986 | DOI:10.1038/s41598-017-08208-2
1
www.nature.com/scientificreports/ Recent studies have described the anterior cingulate cortex (ACC) as a cortical area in the brain involved with pain, possibly including both perception and modulation via neural plasticity18, 19. However, despite studies demonstrating that ACC projection is deeply related to the motor cortex, the underlying mechanism of MCS in pain modulation is poorly understood18. The activation of astrocytes and/or astrogliosis is one of the changes that has been observed in the ACC during chronic pain induced by nerve injury20, 21. In addition, nerve injury manifests as an increased expression of astrocytic markers, such as glial fibrillary acidic protein (GFAP), in the ACC8. Evidence from previous reports shows that astrocytes perform various functions, including the biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to nervous tissue, maintenance of extracellular ion balance, and a role in the repair and scarring process of the brain and spinal cord following traumatic injuries22–24. Astrocytes are the most numerous non-neuronal cells in the brain involved in the modulation of neuronal activities, such as extracellular and synaptic cleft neurotransmitter level regulation and the release of neuroactive molecules18. Moreover, recent studies have suggested that astrocytes play an important role in the synaptic cleft and in the interactions between pain-transmitting neurons and other neurons5, 7. This is because astrocytes can detect neuronal activity and release chemical transmitters, which in turn control synaptic activity19, 22. Thus, we hypothesized that repetitive MCS may induce analgesic effects on chronic neuropathic pain by inducing a modification in synaptic connections, resulting in the attentuation of mechanical hypersensitivity in neuropathic pain. The aim of this study was to examine the altered synaptic connection of the ACC in a rat model of neuropathic pain and MCS-induced behavioral modifications. Our results implicate the potential role of cortical astrocytes and ACC structural synaptic plasticity in mechanical hyperalgesia and contribute to the understanding of the mechanism of MCS-induced analgesic effects in an animal model of neuropathic pain.
Results
Repetitive MCS attenuated pain behavior associated with neuropathic pain. We have previously shown that tibial and sural nerve ligation in rats produced mechanical and thermal hypersensitivity25. The withdrawal threshold and latency induced by electrical von Frey stimulation were measured in peripheral nerve-injured rats. As shown in Fig. 1A, the withdrawal thresholds in neuropathic pain (NP) rats significantly decreased after nerve injury. The measured values of the withdrawal response in NP rats were significantly reduced from the day following nerve injury (P
